Ultrasonic technology, used in the freezing industry, is a good way to effectively improve the quality of frozen products. Physical effects (cavitation effect) of ultrasonic waves can reduce the degree of supercooling needed by ice crystal nucleation to promote the formation of crystal nuclei, especially in an environment with a low degree of supercooling that is difficult for nucleation, where the use of ultrasonic waves can effectively promote nucleation. The ultrasonic waves can break larger ice crystals as well. The bubbles produced by the higher-intensity ultrasonic waves can also act as crystal nuclei effectively, thereby increasing the number of ice crystals, thus reducing the volume of ice crystals. Besides, the phenomenon of strong agitation and micro gasification produced by the ultrasonic cavitation effect can increase the mass and heat transfer coefficient of frozen food, thus achieving the purpose of rapid freezing.
The ultrasonic cavitation effect is influenced by many factors, such as frequency and intensity of the ultrasonic waves, and nature of the object subject to the ultrasonic waves. The ultrasonic waves act on a liquid and causes sonic propagation therein. When the intensity is high enough, there will be a lot of mobile bubbles in the liquid, which are classified into ordinary bubbles and cavitation bubbles in accordance with their vibration behavior. The cavitation bubbles show strong nonlinear effects in the process of movement, and can cause the sonic energy to be converted to other forms of energy. With the increase of the ultrasonic frequency, the cavitation bubbles overall show a decline trend in vibration, and therefore the cavitation effect is correspondingly weakened with the increasing frequency. While with the sonic frequency unchanged, smaller micro bubbles can be converted to the cavitation bubbles within a certain range by increasing the sonic intensity, thereby improving the cavitation effect of the ultrasonic waves.
Da-Wen Sun and Bing Li treated potatoes immersed in the freezing fluid for 2 min in 2002 with ultrasonic waves at a power of 15.85 W and a frequency of 25 kHz, and then observed the potatoes under a scanning electron microscope, finding that the potatoes had intact cell membrane structure and small intercellular space. X. Zhang et al., taking water containing saturated bubbles as an ultrasonic object in the same year, observed that there might be another principle of a tiny stream for the promotion of crystallization by ultrasonic waves, which did not exist in a solid sample, however. Cheng-Hui Wang and Shu-Yu Lin made study on the nonlinear vibration of bubbles under the action of ultrasonic waves in 2010, with the result that the cavitation effect of ultrasonic waves was related to many factors. Hossein Kiani and Da-Wen Sun et al. treated agar gel as a food model with different irradiation time and ultrasonic intensity in 2011, proving that ultrasonic waves could reduce the degree of supercooling needed by crystallization and shorten the freezing time.
Unlike plants, fish may be subject to changes peculiar to animals after being captured or slaughtered. First, the enzyme system of their own starts to work; in glycolysis, glycogen is decomposed into lactic acid, resulting in lower pH. Then, pH drops below a certain level to cause adenosine triphosphatase to start to work; adenosine triphosphate becomes less by being decomposed, and phosphocreatine will also become less until disappear, and then mytolin contracts, which may cause rigor mortis; rigor mortis makes muscles contract, with elasticity and extensibility reduced; on the occasion of fish being caught, the fish will be suffocated if it is in a fishing net, and then glycogen will be decomposed into lactic acid with adenosine triphosphate almost exhausted, here without causing rigor mortis but direct corruption instead. Next, rigor mortis will end before long, with the original hardness restored. Further next, the enzyme in muscles digests itself, thus increasing soluble substances and softening tissue. Finally, with self-digestion increasing the soluble substances, adhering and invading bacteria will proliferate vigorously to cause the decomposed products to accumulate, making the fish flesh corrupted soon.
The patent CN201010130687.2 discloses a method that, by adding a composite antifreezer, can reduce freezing denaturation of the frozen minced fish protein more effectively, and improve gel strength of frozen minced fish. The patent CN201010156877.1 discloses that algin decomposition products, as a phosphorus-free quality-improving agent, can significantly improve quality of fish fillets, and can significantly reduce the product thawing loss and improve the product tenderness, thereby significantly improving the texture of frozen fish fillets. The patent CNO3105265.7 discloses that Chinese medicinal herb and food can be quickly frozen and freshly kept, and ice crystals can be sublimated under freezing conditions, avoiding the phenomenon that substances are oxidized and decomposed due to high-temperature atmospheric evaporation, thus keeping the original specific nutrients and high active ingredient content. The patent CN01135334.1 discloses a food freezing system that adds an electric field to a cooling medium. The patent CN01124944.7 discloses that barrier ice is integrated with thermal insulation materials, thereby increasing the thermal insulation thickness to freeze beet, making the overall input adapted to the output.
With the development of freshwater aquaculture, freshwater fish output increases rapidly. In addition to marketing fresh, it has become one of the methods widely used in the aquatic product processing industry to process the freshwater fish into fish fillets, fish segments and fish steaks, and to freeze them into a small package of frozen food, which has also alleviated the situation of “difficulty in selling” freshwater fish to a certain degree. The cold storage methods include an ice filling process by sprinkling fish with crushed ice, and an ice water cooling process by putting fish into ice water, which have their own characteristics, but cannot generally be used for long-term preservation of fish. Therefore, quick freezing is needed, with the freezing methods generally including a salt-water immersion process (immersing and freezing fish in salt water at about −5° C.), a contact process (freezing fish on a cryopanel at −40° C. to −25° C.), and a half-blasting process (feeding a cold blast at −40° C. to −35° C. at a speed of 3 to 5 m/s for freezing). The freezing storage temperature for freezing fish is preferably as low as possible, especially for a long preservation, but it is typically −20° C. for the economic reasons.
When the fish flesh is stored by being frozen, the water therein is converted from a liquid phase into a solid phase to form ice crystals, which are distributed intracellularly and intercellularly in the fish flesh tissue. The deficiency of the prior art is that, under general freezing conditions, the formed ice crystals tend to be larger than the diameter of fish flesh cells, such that the ice crystals cause physical damage to the cell walls, resulting in protoplasm spillover and cell deformation, and thus the fish flesh loses its original texture, causing a severe nutritional flavor loss after the fish flesh is thawed. Treating the fish flesh with the constant-power ultrasonic waves, due to the different ice crystal growth micro-environment for different parts of the fish flesh, is difficult to achieve a uniform treatment effect.